Secondary battery and electronic device

ABSTRACT

A secondary battery suitable for a portable information terminal or a wearable device is provided. An electronic device having a novel structure which can have various forms and a secondary battery that fits the forms of the electronic device are provided. In the secondary battery, sealing is performed using a film provided with depressions or projections that ease stress on the film due to application of external force. A pattern of depressions or projections is formed on the film by pressing, e.g., embossing.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. One embodiment of the presentinvention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, an imaging device, adriving method thereof, or a manufacturing method thereof. Inparticular, one embodiment of the present invention relates to anelectronic device.

Note that electronic devices in this specification mean all devicesincluding secondary batteries, and electro-optical devices includingsecondary batteries, information terminal devices including secondarybatteries, vehicles including secondary batteries, and the like are allelectronic devices.

BACKGROUND ART

In recent years, portable information terminals typified by smartphoneshave been actively developed. Portable information terminals, which area kind of electronic devices, are desired to be lightweight and compactby users.

Patent Document 1 discloses an example of a hands-free wearable devicewith which information can be visually obtained anywhere, specifically,a goggle-type display device that includes a CPU and is capable of datacommunication. The device disclosed in Patent Document 1 is also a kindof electronic device.

Most wearable devices and portable information terminals includesecondary batteries that can be repeatedly charged and discharged, andhave problems in that there is a limitation on the operation time of thewearable devices and the portable information terminals because theirlight weight and compactness cost the battery capacity. Secondarybatteries used in wearable devices and portable information terminalsshould be lightweight and compact and should be able to be used for along time.

Examples of secondary batteries include a nickel-metal hydride batteryand a lithium-ion secondary battery. In particular, lithium-ionsecondary batteries have been actively researched and developed becausethe capacity thereof can be increased and the size thereof can bereduced.

Electrodes serving as positive electrodes or negative electrodes oflithium-ion secondary batteries are each formed using, for example, alithium metal, a carbon-based material, or an alloy-based material.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2005-157317

Disclosure of Invention

An object is to provide a secondary battery suitable for a portableinformation terminal.

Another object is to provide a secondary battery suitable for a wearabledevice.

Another object is to provide an electronic device having a structurewhich can have various forms and a secondary battery that fits the formsof the electronic device. Another object is to provide a novelelectronic device, a novel secondary battery, or a novel power storagedevice.

Note that the descriptions of these objects do not preclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the descriptions of thespecification, the drawings, the claims, and the like.

In the case where an electronic device is formed to have a complicatedform, a housing is designed to have a complicated form and electroniccomponents (e.g., a power source, a wiring, a transistor, a resistor,and a capacitor) are arranged in an internal space of the housing. Whenit does not matter how large and heavy the electronic device is, thevolume of the internal space of the housing is relatively large; thus,the electronic components can be arranged relatively freely.

In the case where an electronic device having a complicated form isrequired to be compact and lightweight, the volume of an internal spaceof a housing is small, and electronic components and the sizes thereofare selected according to the volume and the electronic components arearranged. In this case, the manufacturing cost is increased becausesmaller electronic components are more expensive.

Moreover, as the volume or weight of a secondary battery increases, thecapacity thereof tends to increase. Therefore, there are limitations onthe size and arrangement of a secondary battery that is incorporated ina small electronic device.

An increase in the mileage per charge of a vehicle using a secondarybattery such as an electric vehicle and a hybrid vehicle increases thevolume or weight of the secondary battery.

In view of the above, a secondary battery that can be changed in form isused for an electronic device and the secondary battery and otherelectronic components are arranged with efficiency in the internal spaceof a housing of the electronic device.

In the case where a secondary battery is changed in form by externallyapplying force, the force is externally applied to an object such as afilm used as an exterior body of the secondary battery and the object isstressed. This might partly deform or damage the object.

A secondary battery that can relieve a strain caused by stress isprovided. A “strain” is the scale of change in form indicating thedisplacement of a point of an object relative to the reference (initial)length of the object. A secondary battery that can reduce the influenceof a strain, that is, the scale of change in form caused by applicationof external force to the secondary battery, to an acceptable level isprovided.

One embodiment of the invention disclosed in this specification is asecondary battery in which sealing is performed using a film providedwith depressions or projections that eases stress on the film due toapplication of external force.

Another embodiment of the invention disclosed in this specification is asecondary battery in which sealing is performed using a film having apattern of depressions or projections on part of a surface.

Another embodiment of the invention disclosed in this specification is asecondary battery in which sealing is performed by attaching a firstfilm having a pattern of depressions or projections on part of a surfaceto a second film having a pattern of depressions or projectionsdifferent from that of the first film on part of a surface.

(1) Another embodiment of the invention disclosed in this specificationis a secondary battery including a film that is used for sealing. Thefilm has a region with a pattern of depressions or projections and aregion without the pattern of depressions or projections. The regionwith the pattern includes a portion having a first thickness and aportion having a second thickness.

(2) Another embodiment of the invention disclosed in this specificationis a secondary battery including a film that is used for sealing. Thefilm has a region with a pattern of depressions or projections and aregion without the pattern of depressions or projections. The regionwith the pattern includes a portion having a first thickness and aportion having a second thickness. The portion having the firstthickness is provided with depressions or projections with a first pitchand the portion having the second thickness is provided with depressionsor projections with a second pitch. Note that a “pitch” refers to thedistance between bottoms of adjacent depressions or tops of adjacentprojections seen from the top.

(3) Another embodiment of the invention disclosed in this specificationis the secondary battery of (1) or (2) including a boundary between theregion with the pattern of depressions or projections and the regionwithout the pattern of depressions or projections.

(4) Another embodiment of the invention disclosed in this specificationis a secondary battery including a film that is used for sealing. Thefilm has a first pattern of depressions or projections. The filmincludes a portion having a first thickness and a portion having asecond thickness. The film has a second pattern of depressions orprojections. The film includes a portion having a third thickness and aportion having a fourth thickness. The first pattern is formed by theportion having the first thickness and the portion having the secondthickness. The second pattern is formed by the portion having the thirdthickness and the portion having the fourth thickness.

(5) Another embodiment of the invention disclosed in this specificationis a secondary battery including a film that is used for sealing. Thefilm has a first pattern of depressions or projections. The filmincludes a portion having a first thickness and a portion having asecond thickness. The film has a second pattern of depressions orprojections. The film includes a portion having a third thickness and aportion having a fourth thickness. The portion having the firstthickness is provided with depressions or projections with a firstpitch. The portion having the third thickness is provided withdepressions or projections with a second pitch.

(6) Another embodiment of the invention disclosed in this specificationis the secondary battery of (4) or (5) including a boundary between thefirst pattern of depressions or projections and the second pattern ofdepressions or projections.

(7) Another embodiment of the invention disclosed in this specificationis a secondary battery including a first film and a second film that areused for sealing. The first film has a region with a pattern ofdepressions or projections and the second film has a region without thepattern of depressions or projections. The region with the patternincludes a portion having a first thickness and a portion having asecond thickness.

(8) Another embodiment of the invention disclosed in this specificationis a secondary battery including a first film and a second film that areused for sealing. The first film has a region with a pattern ofdepressions or projections and the second film has a region without thepattern of depressions or projections. The region with the patternincludes a portion having a first thickness and a portion having asecond thickness. The portion having the first thickness is providedwith depressions or projections with a first pitch and the portionhaving the second thickness is provided with depressions or projectionswith a second pitch.

(9) Another embodiment of the invention disclosed in this specificationis the secondary battery of (7) or (8) including at least a positiveelectrode active material layer, a negative electrode active materiallayer, and an electrolyte solution between the first film and the secondfilm.

(10) Another embodiment of the invention disclosed in this specificationis the secondary battery of (2) or (8) in which the first pitch isdifferent from the second pitch.

(11) Another embodiment of the invention disclosed in this specificationis the secondary battery of (2) or (8) in which the depths ofdepressions with the first pitch or the heights of projections with thefirst pitch and the depths of depressions with the second pitch or theheights of projections with the second pitch are smaller than half thethickness of the secondary battery.

(12) Another embodiment of the invention disclosed in this specificationis a secondary battery including a first film and a second film that areused for sealing. The first film has a first pattern of depressions orprojections. The first film includes a portion having a first thicknessand a portion having a second thickness. The second film has a secondpattern of depressions or projections. The second film includes aportion having a third thickness and a portion having a fourththickness. The first pattern is formed by the portion having the firstthickness and the portion having the second thickness. The secondpattern is formed by the portion having the third thickness and theportion having the fourth thickness.

(13) Another embodiment of the invention disclosed in this specificationis a secondary battery including a first film and a second film that areused for sealing. The first film has a first pattern of depressions orprojections. The first film includes a portion having a first thicknessand a portion having a second thickness. The second film has a secondpattern of depressions or projections. The second film includes aportion having a third thickness and a portion having a fourththickness. The portion having the first thickness is provided withdepressions or projections with a first pitch. The portion having thethird thickness is provided with depressions or projections with asecond pitch.

(14) Another embodiment of the invention disclosed in this specificationis the secondary battery of (12) or (13) including at least a positiveelectrode active material layer, a negative electrode active materiallayer, and an electrolyte solution between the first film and the secondfilm.

(15) Another embodiment of the invention disclosed in this specificationis the secondary battery of (5) or (13) in which the first pattern ofdepressions or projections is formed by the portion having the firstthickness and the portion having the second thickness. The secondpattern of depressions or projections is formed by the portion havingthe third thickness and the portion having the fourth thickness.

(16) Another embodiment of the invention disclosed in this specificationis the secondary battery of (5) or (13) in which the depths ofdepressions with the first pitch or the heights of projections with thefirst pitch and the depths of depressions with the second pitch or theheights of projections with the second pitch are smaller than half thethickness of the secondary battery.

In any of the above structures, the pattern of the film is a visuallyrecognizable geometric pattern in which lines slanted in two directionscross each other. In the case of such a geometric pattern in which linesslanted in two directions cross each other, stress due to bending can berelieved in at least two directions. The depressions or projections arenot necessarily arranged regularly and may be arranged randomly. Randomarrangement enables stress due to two-dimensional bending, stress due tothree-dimensional random bending, or stress due to twisting to berelieved. The film may include a plurality of regions having differentpatterns. For example, the film may be provided with different patternsin the end portion and at the center, providing one film with two typesof patterns. Alternatively, the film may be provided with three or moretypes of patterns. The film may be provided with depressions orprojections only in a bendable portion and may have a flat surface inthe other portion. Note that there is no particular limitation on theshapes of depressions or projections.

The depressions or projections of the film can be formed by pressing(e.g., embossing). The depressions or projections formed on a surface(or on the back) of the film by embossing form an enclosed space that issealed by the film serving as a part of a wall of the sealing structureand whose inner volume is variable. This enclosed space can be regardedto have an accordion structure (bellows structure) formed by the filmwith the pattern of depressions or projections. The sealing structureusing the film can prevent entry of water and dust. Note that embossing,which is a kind of pressing, is not necessarily employed and a methodthat allows formation of a relief on part of the film can be employed. Acombination of the methods, for example, embossing and any otherpressing may be performed on one film. Alternatively, embossing may beperformed on one film more than once.

Although the secondary battery can have any of a variety of structures,a structure where a film is used as an exterior body is employed here.The film needs to have water resistance and gas resistance. Note thatthe film used as the exterior body is a single-layer film selected froma metal film (e.g., a foil of a metal or an alloy such as aluminum,stainless steel, nickel steel, gold, silver, copper, titanium, nichrome,iron, tin, tantalum, niobium, molybdenum, zirconium, or zinc), a plasticfilm made of an organic material, a hybrid material film containing anorganic material (e.g., an organic resin or fiber) and an inorganicmaterial (e.g., ceramic), and a carbon-containing inorganic film (e.g.,a carbon film or a graphite film) or a stacked-layer film including twoor more of the above films. A metal film is easy to be embossed. Formingdepressions or projections by embossing increases the surface area ofthe film exposed to outside air, achieving efficient heat dissipation.

The sealing structure of the secondary battery is as follows: onerectangular film is folded in half such that two end portions overlapwith each other and is sealed on three sides with an adhesive layer, ortwo films are stacked so as to overlap with each other and is sealed onfour sides, which are end portions of the film, with an adhesive layer.

The adhesive layer can be formed using a thermoplastic film material, athermosetting adhesive, an anaerobic adhesive, a photo-curable adhesivesuch as a UV curable adhesive, or a reactive curable adhesive. Examplesof materials of the adhesives include an epoxy resin, an acrylic resin,a silicone resin, and a phenol resin.

In forming the sealing structure by bonding and fixing the adhesivelayer and the film, pressure bonding is performed. The depths of thedepressions or the heights of the projections are made different betweenan end portion of the film, which is subjected to pressure bonding, anda center portion of the film. When the depths of the depressions or theheights of the projections in the end portion of the film are smallerthan the depths of the depressions or the heights of the projections inthe center portion of the film, the influence of a strain can be reducedto be within the allowable range.

In the case where a film is provided with depressions or projections ina center portion and not provided with depressions or projections in anend portion subjected to pressure bonding, the secondary battery cangreatly expand when the volume of internal components of the secondarybattery expands in the center portion. That is, such a film can preventexplosion of the secondary battery. On the other hand, because ofabsence of depressions or projections in the end portion, flexibility ofthe end portion is lower than that in the center portion and stress isless likely to be relieved in the end portion than in the centerportion. Therefore, providing depressions or projections also in the endportion of the film helps reduce the influence of a strain to anacceptable level.

The term “electronic device having a complicated form” can beinterpreted in many ways. It can be interpreted as an electronic devicehaving a fixed complicated form (e.g., the form having a curvedsurface). In the case of fixing the form of the electronic device, asecondary battery is bent once and fixed while being bent. In addition,the term can also be interpreted as an electronic device having acomplicated form that changes or does not change its form when externalforce is applied or an electronic device having a simple form thatchanges its form when external force is applied. In the case of anelectronic device that changes its form when force is applied, it ispreferable that a secondary battery also be able to change its formevery time force is applied.

One embodiment of the invention disclosed in this specification is anelectronic device including a housing partly having a curved surface anda secondary battery having a curved surface. An exterior body of thesecondary battery is a film whose surface partly has a pattern formed bydepressions or projections.

Another embodiment of the invention disclosed in this specification isan electronic device including a housing and a secondary battery incontact with part of the housing. An exterior body of the secondarybattery is a film whose surface partly has a pattern formed bydepressions or projections. The housing can partly change its form.

In the above structure, the exterior body of the secondary battery canchange its form in the range of radius of curvature from 10 mm to 150mm, preferably from 30 mm to 150 mm. One or two films are used as theexterior body of the secondary battery. In the case where the secondarybattery has a layered structure and the secondary battery has anarc-formed cross section by bending the secondary battery, the secondarybattery has a structure where electrodes, an electrolyte solution, andthe like are sandwiched between two curved surfaces of the films.

A description is given of the radius of curvature of a surface withreference to FIGS. 1A to 1C. In FIG. 1A, on a plane 1701 along which acurved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius of curvature 1703 and the center of thecircle is referred to as a center of curvature 1704. FIG. 1B is a topview of the curved surface 1700. FIG. 1C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut by a plane, the radius of curvature of a curve in a crosssection differs depending on the angle between the curved surface andthe plane or on the cut position, and the smallest radius of curvatureis defined as the radius of curvature of a surface in this specificationand the like.

In the case of bending a secondary battery in which electrodes and anelectrolyte solution 1805 and the like are sandwiched between two filmsas exterior bodies, a radius 1802 of curvature of a film 1801 close to acenter of curvature 1800 of the secondary battery is smaller than aradius 1804 of curvature of a film 1803 far from the center of curvature1800 (FIG. 2A). When the secondary battery is bent and has an arc-shapedcross section, tensile stress is applied to a surface of the film on theside farther from the center of curvature 1800 (FIG. 2B). However, byforming a pattern including projections or depressions on surfaces ofthe exterior bodies, the influence of a strain can be reduced to beacceptable even when tensile stress is applied. For this reason, thesecondary battery can change its form such that the exterior body on theside closer to the center of curvature has a curvature radius greaterthan or equal to 10 mm, preferably greater than or equal to 30 mm.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped as illustrated in FIG. 2C.

When the bent secondary battery is seen in cross section, the outsidesurface is stretched and the inside surface is compressed. In otherwords, the outside surface expands and the inside surface contracts.

An optimum pattern of depressions or projections formed on a film thatis the exterior body can prevent deterioration or breakage of the filmdue to a wrinkle or a crack formed by bending of a secondary battery andthus can prevent leakage of an electrolyte solution.

Part of a device like a watch is brought in contact with part of thebody (wrist or arm) of a user, that is, the user wears the device,whereby the user can feel like the device is lighter than the actualweight. The use of a flexible secondary battery in an electronic devicehaving a form with a curved surface that fits part of the body of a userallows the secondary battery to be fixed so as to have a form suitableto the electronic device and provided.

When a user moves part of the body on which an electronic device isworn, the user might feel uncomfortable, regard the electronic device asa distraction, and feel stress even in the case where the electronicdevice has a curved surface that fits part of the body. In the casewhere at least part of an electronic device can be changed in formaccording to movement of a body of a user, the user does not feeluncomfortable and a flexible battery can be provided in a portion of theelectronic device that can be changed in form.

An electronic device does not necessarily have a form with a curvedsurface or a complicated form; an electronic device may have a simpleform. For example, the number or size of components that can beincorporated in an electronic device with a simple form is determineddepending on the volume of a space formed by a housing of the electronicdevice in many cases. Providing a flexible secondary battery in a smallspace between components other than the secondary battery enables aspace formed by a housing of an electronic device to be efficientlyused; thus, the electronic device can be reduced in size.

Examples of wearable devices include wearable input terminals such as awearable camera, a wearable microphone, and a wearable sensor, wearableoutput terminals such as a wearable display and a wearable speaker, andwearable input/output terminals having the functions of any of the inputterminals and any of the output terminals. Another example of a wearabledevice is a device that controls each device and calculates or processesdata, typically, a wearable computer including a CPU. Other examples ofwearable devices include devices that store data, send data, and receivedata, typically, a portable information terminal and a memory.

A secondary battery having a novel structure can be provided. A novelpower storage device can be provided.

The form of a secondary battery can be freely designed and when asecondary battery having a curved surface is used for example, thedesign flexibility of a whole device is increased and devices having avariety of designs can be fabricated. Furthermore, a secondary batteryis provided inside and along a curved surface of a device with the leastwasted space in the device having the curved surface, whereby it ispossible to make maximum use of a space in the device.

Thus, an electronic device having a novel structure can be provided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot have to achieve all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C illustrate a radius of curvature of a surface.

FIGS. 2A to 2C illustrate a center of curvature.

FIGS. 3A to 3C are top views each illustrating one embodiment of thepresent invention.

FIGS. 4A and 4B are top views each illustrating one embodiment of thepresent invention.

FIGS. 5A to 5F illustrate embossing of one embodiment of the presentinvention.

FIGS. 6A to 6F are top views, a cross-sectional view, and a schematicview illustrating one embodiment of the present invention.

FIGS. 7A to 7C are top views and a cross-sectional view illustrating oneembodiment of the present invention.

FIGS. 8A to 8D are each a cross-sectional view of a secondary battery ofone embodiment of the present invention.

FIGS. 9A to 9C are each a cross-sectional view of a secondary battery ofone embodiment of the present invention.

FIGS. 10A and 10B are top views each illustrating one embodiment of thepresent invention.

FIGS. 11A to 11E are top views each illustrating one embodiment of thepresent invention.

FIGS. 12A to 12C are perspective views and a cross-sectional view eachillustrating one embodiment of the present invention.

FIGS. 13A to 13C are perspective views and a cross-sectional view eachillustrating one embodiment of the present invention.

FIGS. 14A to 14H illustrate electronic devices including flexiblesecondary batteries.

FIGS. 15A to 15C illustrate an electronic device.

FIGS. 16A and 16B illustrate vehicles including secondary batteries.

FIGS. 17A to 17C are cross-sectional views and a photograph forexplaining Example 1.

FIG. 18 is a graph showing friction data in Example 1.

FIGS. 19A to 19D illustrate Example 2.

FIG. 20 illustrates Example 2.

FIGS. 21A to 21C are photographs showing Example 2.

FIGS. 22A and 22B illustrate Example 3.

FIGS. 23A and 23B illustrate Examples 3 and 4.

FIG. 24 is a graph showing data of a load test in Example 3.

FIGS. 25A and 25B are each a graph showing data of a load test inExample 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and examples of the present invention will be describedbelow in detail with reference to the drawings. However, the presentinvention is not limited to the descriptions below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Further, the present inventionis not construed as being limited to the descriptions of the embodimentsand the examples.

The term “electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on the “object having any electric function” aslong as electric signals can be transmitted and received between thecomponents connected through the object.

The position, size, range, or the like of each component illustrated indrawings and the like is not accurately represented in some cases forsimplification. Therefore, the disclosed invention is not necessarilylimited to the position, size, range, or the like disclosed in thedrawings and the like.

The ordinal number such as “first”, “second”, and “third” are used toavoid confusion among components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. In addition, the term “insulating film” can be changed intothe term “insulating layer” in some cases.

Embodiment 1

In this embodiment, an example of fabricating a lithium-ion secondarybattery with the use of a film having an embossed pattern will bedescribed with reference to FIGS. 3A to 3C, FIGS. 4A and 4B, FIGS. 5A to5F, FIGS. 6A to 6F, FIGS. 7A to 7C, FIGS. 8A to 8D, and FIGS. 9A to 9C.

First, a sheet made of a flexible material is prepared. As the sheet, astacked body, a metal film provided with an adhesive layer (alsoreferred to as a heat-seal layer) or sandwiched between adhesive layersis used. As the adhesive layer, a heat-seal resin film containing, e.g.,polypropylene or polyethylene is used. In this embodiment, a metalsheet, specifically, aluminum foil whose top surface is provided with anylon resin and whose bottom surface is provided with a stack includingan acid-resistant polypropylene film and a polypropylene film is used asthe sheet. This sheet is cut to obtain a film 10 illustrated in FIG. 3A.

A film 10 a of the film 10 is embossed and a film 10 b is not embossed;as a result, a film 11 illustrated in FIG. 3B is formed. As illustratedin FIG. 3B, projections and depressions are formed to provide a film 11a a surface of which is provided with a visually recognizable patternand a film 11 b a surface of which is not provided with projections anddepressions. There is a boundary between the film 11 a provided withprojections and depressions and the film 11 b not provided withprojections and depressions. In FIG. 3B, the film 11 a is an embossedportion of the film 11, and the film 11 b is a non-embossed portion.Note that embossing for the film 11 a may be performed to provide thesame projections and depressions on the entire surface, or may beperformed to provide two or more types of projections and depressions onthe film 11 a. In the latter case, a boundary is formed between any twodifferent types of projections and depressions.

Alternatively, the entire film 10 in FIG. 3A may be embossed to form afilm 22 as illustrated in FIG. 4A. Note that embossing may be performedto provide the same projections and depressions on the entire film 22,or may be performed to provide two or more types of projections anddepressions on the film 22. In the latter case, a boundary is formedbetween any two different types of projections and depressions.

Although an example where the sheet is cut and then embossing isperformed is described here, there is no particular limitation on theorder; embossing may be performed before cutting the sheet and then thesheet is cut so as to be in the state illustrated in FIG. 3B.Alternatively, the sheet may be cut after thermocompression bonding isperformed with the sheet bent.

Embossing, which is a kind of pressing, will be described.

FIGS. 5A to 5F are cross-sectional views showing examples of embossing.Note that embossing refers to processing for forming projections anddepressions on a film by bringing an embossing roll whose surface hasprojections and depressions into contact with the film with pressure.The embossing roll is a roll whose surface is patterned.

An example where one surface of a film is embossed is illustrated inFIG. 5A.

FIG. 5A illustrates the state where a film 50 is sandwiched between anembossing roll 53 in contact with the one surface of the film and a roll54 in contact with the other surface and the film 50 is beingtransferred in a direction 60. The surface of the film is patterned bypressure or heat. The surface of the film may be patterned by pressureand heat.

Processing illustrated in FIG. 5A is called one-side embossing, whichcan be performed by a combination of the embossing roll 53 and the roll54 (a metal roll or an elastic roll such as a rubber roll).

FIG. 5B illustrates the state where a film 51 whose one surface isembossed is sandwiched between the embossing roll 53 and the roll 54 andis transferred in the direction 60. The embossing roll 53 rolls along anon-embossed surface of the film 51; thus, both surfaces of the film 51are embossed. As described here, one film can be embossed plural times.

FIG. 5C is an enlarged view of a cross section of a film 52 whose bothsurfaces are embossed. Note that H₁ represents the thickness of the filmin depressions or projections, and H₂ represents the thickness of thefilm at a boundary portion between a depression and its adjacentprojection or the thickness of the film at a boundary portion between aprojection and its adjacent depression. The thickness of the film is notuniform, and H₂ is smaller than H₁.

FIG. 5D illustrates another example where both surfaces of a film areembossed.

FIG. 5D illustrates the state where the film 50 is sandwiched betweenthe embossing roll 53 in contact with one surface of the film and anembossing roll 55 in contact with the other surface and the film 50 isbeing transferred in the direction 60.

Processing illustrated in FIG. 5D is called both-side embossing, whichcan be performed by a combination of the embossing roll 53 and theembossing roll 55, in which a depression of one embossing roll and aprojection of the other embossing roll are in a set. The surface of thefilm 50 is patterned with alternate projections and depressions;projections are formed by raising part of the surface of the film 50 anddepressions are formed by concaving part of the surface of the film 50.

FIG. 5E illustrates the case of using the embossing roll 53 and anembossing roll 56 for making protrusions with a pitch different fromthat of protrusions made with the embossing roll 53 in FIG. 5D. Notethat a protrusion pitch or an embossing pitch is the distance betweenthe tops of adjacent protrusions. For example, a distance P in FIG. 5Eis a protrusion pitch or an embossing pitch. FIG. 5E illustrates thestate where the film 50 is sandwiched between the embossing roll 53 andthe embossing roll 56 and is transferred in the direction 60. The filmprocessed using the embossing rolls with different protrusion pitchescan have surfaces with different embossing pitches.

FIG. 5F illustrates the state where the film 50 is sandwiched between anembossing roll 57 in contact with one surface of the film and anembossing roll 58 in contact with the other surface and the film 50 isbeing transferred in the direction 60.

Processing illustrated in FIG. 5F is called tip-to-tip both-sideembossing performed by a combination of the embossing roll 57 and theembossing roll 58 that has the same pattern as the embossing roll 57.The phases of the projections and depressions of the two embossing rollsare the same, so that substantially the same pattern can be formed onboth surfaces of the film 50. Unlike in the case of FIG. 5F, embossingmay be performed without matching the phases of the projections anddepressions of the same embossing rolls.

An embossing plate can be used instead of the embossing roll.Furthermore, embossing is not necessarily employed, and any method thatallows formation of a relief on part of the film can be employed.

In this embodiment, projections and depressions are provided on bothsurfaces of the film 10 a of the film 10 so that the film 11 havingpatterns is formed, and the film 11 is folded at the center such thattwo end portions overlap with each other, and is sealed on three sideswith an adhesive layer.

Then, the film 11 is folded along a dotted line shown in FIG. 3B so asto be in the state illustrated in FIG. 6A.

A positive electrode current collector 12 on the surface of which apositive electrode active material layer 18 is partly formed, aseparator 13, and a negative electrode current collector 14 on thesurface of which a negative electrode active material layer 19 is partlyformed are stacked as illustrated in FIG. 6B to constitute a secondarybattery. The positive electrode current collector 12 and the negativeelectrode current collector 14 can each be formed using a highlyconductive material that is not alloyed with a carrier ion such as alithium ion, for example, a metal such as stainless steel, gold,platinum, zinc, iron, nickel, copper, aluminum, titanium, and tantalumor an alloy thereof. Alternatively, an aluminum alloy to which anelement which improves heat resistance, such as silicon, titanium,neodymium, scandium, and molybdenum, is added can be used. Stillalternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The current collectors can each have a foil-like shape, aplate-like shape (sheet-like shape), a net-like shape, a cylindricalshape, a coil shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate. The current collectors each preferably have athickness of 5 μm to 40 μm inclusive. Note that in the exampleillustrated here, for simplicity, one stack including the positiveelectrode current collector 12 provided with the positive electrodeactive material layer 18, the separator 13, and the negative electrodecurrent collector 14 provided with the negative electrode activematerial layer 19 is packed in an exterior body. To increase thecapacity of a secondary battery, a plurality of the stacks may bestacked and packed in an exterior body.

In addition, two lead electrodes 16 with sealing layers 15 illustratedin FIG. 6C are prepared. The lead electrodes 16 are each also referredto as a lead terminal and provided in order to lead a positive electrodeor a negative electrode of a secondary battery to the outside of anexterior film. Aluminum and nickel-plated copper are used for thepositive electrode lead and the negative electrode lead, respectively.

Then, the positive electrode lead is electrically connected to aprotruding portion of the positive electrode current collector 12 byultrasonic welding or the like, and the negative electrode lead iselectrically connected to a protruding portion of the negative electrodecurrent collector 14 by ultrasonic welding or the like.

Then, two sides of the film 11 are sealed by thermocompression bonding,and one side is left open for introduction of an electrolyte solution(hereinafter the shape of a film in this state also referred to as aform of a bag). In thermocompression bonding, the sealing layers 15provided on the lead electrodes are also melted, thereby fixing the leadelectrodes and the film 11 to each other. After that, in reducedpressure or an inert gas atmosphere, a desired amount of electrolytesolution is introduced to the inside of the film 11 in the form of abag. Lastly, the outer edge of the film 11 that has not been subjectedto thermocompression bonding and is left open is sealed bythermocompression bonding.

In this manner, a secondary battery 40 illustrated in FIG. 6D can befabricated.

In the obtained secondary battery 40, the surface of the film 10 servingas an exterior body has a pattern of projections and depressions. Aregion between a dotted line and an edge in FIG. 6D is athermocompression-bonded region 17. A surface of thethermocompression-bonded region 17 also has a pattern of projections anddepressions. Although the heights of projections and the depths ofdepressions are smaller in the thermocompression-bonded region 17 thanin the center portion, the projections and depressions can relievestress applied when the secondary battery is bent.

FIG. 6E illustrates an example of a cross section taken alongdashed-dotted line A-B in FIG. 6D.

As illustrated in FIG. 6E, projections and depressions of the film 11 aare different between a region overlapping with the positive electrodecurrent collector 12 and the thermocompression-bonded region 17. Asillustrated in FIG. 6E, the positive electrode current collector 12, thepositive electrode active material layer 18, the separator 13, thenegative electrode active material layer 19, and the negative electrodecurrent collector 14 are stacked in this order and placed inside thefolded film 11, an end portion is sealed with an adhesive layer 30, andthe other space inside the folded film 11 is provided with anelectrolyte solution 20.

In a similar manner, the secondary battery 40 illustrated in FIG. 7B maybe fabricated using the film 22, which is formed by entirely providingprojections and depressions on both the surfaces of the film 10 to forma pattern. FIG. 7A illustrates the film 22 folded along a line indicatedby a dotted line in FIG. 4A.

FIG. 7C illustrates an example of a cross section of the secondarybattery 40 formed using the film 22 taken along dashed-dotted line E-Fin FIG. 7B.

As illustrated in FIG. 7C, projections and depressions of the film 22are different between a region overlapping with the positive electrodecurrent collector 12 and the thermocompression-bonded region 17. Asillustrated in FIG. 7C, the positive electrode current collector 12, thepositive electrode active material layer 18, the separator 13, thenegative electrode active material layer 19, and the negative electrodecurrent collector 14 are stacked in this order and placed inside thefolded film 22, an end portion is sealed with an adhesive layer 30, andthe other space inside the folded film 22 is provided with anelectrolyte solution 20.

Examples of positive electrode active materials that can be used for thepositive electrode active material layer 18 include a composite oxidewith an olivine structure, a composite oxide with a layered rock-saltstructure, and a composite oxide with a spinel structure. Specifically,a compound such as LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂can be used.

Alternatively, a complex material (LiMPO₄ (general formula) (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II))) can be used. Typicalexamples of the general formula LiMPO₄ which can be used as a materialare lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(j)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Alternatively, a complex material such as Li_((2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≤j≤2)may be used. Typical examples of the general formula Li_((2-j))MSiO₄which can be used as a material are lithium compounds such asLi_((2-j))FeSiO₄, Li_((2-j))NiSiO₄, Li_((2-j))CoSiO₄, Li_((2-j))MnSiO₄,Li_((2-j))Fe_(k)Ni_(l)SiO₄, Li_((2-j))Fe_(k)Co_(l)SiO₄,Li_((2-j))Fe_(k)Mn_(l)SiO₄, Li_((2-j))Ni_(k)Co_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1, and 0<l<1),Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si, and x≥2) can be used for the positive electrode activematerial. Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃,and Li₃Fe₂(PO₄)₃. Further alternatively, a compound expressed byLi₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), aperovskite fluoride such as NaFeF₃ and FeF₃, a metal chalcogenide (asulfide, a selenide, or a telluride) such as TiS₂ and MoS₂, an oxidewith an inverse spinel structure such as LiMVO₄, a vanadium oxide (V₂O₅,V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organic sulfur, orthe like can be used as the positive electrode active material.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a material containing an alkalimetal (e.g., sodium or potassium) or an alkaline-earth metal (e.g.,calcium, strontium, barium, beryllium, or magnesium) instead of lithiummay be used as the positive electrode active material.

As the separator 13, an insulator such as cellulose (paper),polyethylene with pores, and polypropylene with pores can be used.

As an electrolyte in the electrolyte solution, a material having carrierion mobility and containing lithium ions serving as carrier ions isused. Typical examples of the electrolyte are lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, and Li(C₂F₅SO₂)₂N.One of these electrolytes may be used alone, or two or more of them maybe used in an appropriate combination and in an appropriate ratio.

As a solvent of the electrolyte solution, a material with the carrierion mobility is used. As the solvent of the electrolyte solution, anaprotic organic solvent is preferably used. Typical examples of aproticorganic solvents include ethylene carbonate (EC), propylene carbonate,dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, tetrahydrofuran, and the like, and one ormore of these materials can be used. When a gelled high-molecularmaterial is used as the solvent of the electrolyte solution, safetyagainst liquid leakage and the like is improved. Furthermore, thestorage battery can be thinner and more lightweight. Typical examples ofgelled high-molecular materials include a silicone gel, an acrylic gel,an acrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a gel of a fluorine-based polymer, and the like.Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolyte solution can prevent thestorage battery from exploding or catching fire even when the storagebattery internally shorts out or the internal temperature increasesowing to overcharging and others. An ionic liquid is a salt in theliquid state and has high ion mobility (conductivity). An ionic liquidcontains a cation and an anion. Examples of ionic liquids include anionic liquid containing an ethylmethylimidazolium (EMI) cation and anionic liquid containing an N-methyl-N-propylpiperidinium (PP₁₃) cation.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

A material with which lithium can be dissolved and precipitated or amaterial into and from which lithium ions can be inserted and extractedcan be used for a negative electrode active material of the negativeelectrode active material layer 19; for example, a lithium metal, acarbon-based material, an alloy-based material, or the like can be used.

The lithium metal is preferable because of its low redox potential(−3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, fullerene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.1 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferable because of its advantages such asrelatively high capacity per unit volume, small volume expansion, lowcost, and safety greater than that of a lithium metal.

For the negative electrode active material, an alloy-based material oran oxide which enables charge-discharge reactions by an alloyingreaction and a dealloying reaction with lithium can be used. In the casewhere carrier ions are lithium ions, a material containing at least oneof Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, Ga, and the like canbe used as such an alloy-based material, for example. Such elements havehigher capacity than carbon. In particular, silicon has a significantlyhigh theoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material. Examples ofthe alloy-based material using such elements include Mg₂Si, Mg₂Ge,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Note that SiOrefers to the powder of a silicon oxide including a silicon-rich portionand can also be referred to as SiO_(y) (2>y>0). Examples of SiO includea material containing one or more of Si₂O₃, Si₃O₄, and Si₂O and amixture of Si powder and silicon dioxide (SiO₂). Furthermore, SiO maycontain another element (e.g., carbon, nitrogen, iron, aluminum, copper,titanium, calcium, and manganese). In other words, SiO refers to acolored material containing two or more of single crystal silicon,amorphous silicon, polycrystal silicon, Si₂O₃, Si₃O₄, Si₂O, and SiO₂.Thus, SiO can be distinguished from SiO_(x), which is clear andcolorless or white. Note that in the case where a secondary battery isfabricated using SiO as a material thereof and the SiO is oxidizedbecause of repeated charge and discharge cycles, SiO is changed intoSiO₂ in some cases.

Alternatively, for the negative electrode active materials, an oxidesuch as SiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used.

Still alternatively, for the negative electrode active materials,Li_((3-x))M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Note that any of the fluorides can be used as a positive electrodeactive material because of its high potential.

The negative electrode active material layer 19 may further include abinder for increasing adhesion of active materials, a conductiveadditive for increasing the conductivity of the negative electrodeactive material layer 19, and the like in addition to the above negativeelectrode active materials.

In the secondary battery, for example, the separator 13 has a thicknessof approximately 15 μm to 30 μm, the positive electrode currentcollector 12 has a thickness of approximately 10 μm to 40 μm, thepositive electrode active material layer 18 has a thickness ofapproximately 50 μm to 100 μm, the negative electrode active materiallayer 19 has a thickness of approximately 50 μm to 100 μm, and thenegative electrode current collector 14 has a thickness of approximately5 μm to 40 μm. The film 11 has a thickness of approximately 0.113 mm.The film 11 is embossed to a depth of approximately 500 μm. If the film11 is embossed to a depth of 2 mm or more, the whole secondary batteryis too thick.

The battery capacity per unit volume is preferably as large as possible.The battery capacity per unit volume becomes large as the proportion ofthe volume of a battery portion to the total volume of the secondarybattery increases. When the embossing depth is increased, the totalthickness of the secondary battery is increased and the proportion ofthe volume of the battery portion to the total volume is decreased,resulting in a small battery capacity.

The proportion of the volume of the battery portion to the total volumeof the secondary battery is preferably greater than or equal to 50%.FIG. 8A is an example of a cross-sectional view of the secondary batteryin FIG. 6D taken along line C-D. FIG. 9A is an example of across-sectional view of the secondary battery in FIG. 7B taken alongline G-H. FIG. 8A and FIG. 9A each illustrate components 70 in thebattery, an embossed film 71 that covers the top surface of the battery,and a non-embossed film 72 or an embossed film 72 that covers the bottomsurface of the battery. For simplification of the drawings, theelectrolyte solution and a stack including the positive electrodecurrent collector on the surface of which the positive electrode activematerial layer is formed, the separator, the negative electrode currentcollector on the surface of which the negative electrode active materiallayer is formed, and the like are collectively illustrated as thecomponents 70 in the battery. Note that T represents the thickness ofthe components 70 in the battery, t₁ represents the summation of theembossing depth and the thickness of the embossed film 71 that coversthe top surface of the battery, and t₂ represents the thickness of thenon-embossed film 72 that convers the bottom surface of the battery orthe summation of the embossing depth and the thickness of the embossedfilm 72 that convers the bottom surface of the battery. The totalthickness of the secondary battery can be expressed by T+t₁+t₂. Thismeans that T>t₁+t₂ needs to be satisfied to make the proportion of thevolume of the components 70 in the battery to the total volume of thesecondary battery greater than or equal to 50%.

The adhesive layer 30, which is only partly illustrated in FIG. 6E andFIG. 7C, is formed in the following manner: a layer made ofpolypropylene is provided on the entire surface of the film on the sidewhere the attachment is performed, and only a thermocompression-bondedportion becomes the adhesive layer 30.

FIG. 6E and FIG. 7C each illustrate an example where the film 11 b orthe bottom side of the film 22 is fixed and pressure bonding isperformed. In this case, the top side is greatly bent and a step isformed. Thus, when a plurality of combinations of the above stackedlayers (e.g., eight or more combinations) is provided inside the foldedfilm 11 or 22, the step is large and the film 11 a or the top side ofthe film 22 might be too stressed. Furthermore, an edge portion of thetop side of the film might be misaligned with an edge portion of thebottom side of the film. To prevent misalignment of the edge portions, astep may also be provided for the bottom side of the film and pressurebonding may be performed at a center portion so that stress is uniformlyapplied.

In the case where the misalignment is large, there is a region wherepart of an end portion of one film does not overlap with the other film.To correct the misalignment of the end portions of the upper film andthe lower film, such a region may be cut off.

Here, a current flow in charging a secondary battery will be describedwith reference to FIG. 6F. When a secondary battery using lithium as acarrier ion is regarded as a closed circuit, lithium ions transfer and acurrent flows in the same direction. Note that in the secondary batteryusing lithium, an anode and a cathode change places in charge anddischarge, and an oxidation reaction and a reduction reaction occur onthe corresponding sides; hence, an electrode with a high redox potentialis called a positive electrode and an electrode with a low redoxpotential is called a negative electrode. For this reason, in thisspecification, the positive electrode is referred to as a “positiveelectrode” and the negative electrode is referred to as a “negativeelectrode” in all the cases where charge is performed, discharge isperformed, a reverse pulse current is supplied, and a charging currentis supplied. The use of the terms “anode” and “cathode” related to anoxidation reaction and a reduction reaction might cause confusionbecause the anode and the cathode change places at the time of chargingand discharging. Thus, the terms “anode” and “cathode” are not used inthis specification. If the term “anode” or “cathode” is used, it shouldbe mentioned that the anode or the cathode is which of the one at thetime of charging or the one at the time of discharging and correspondsto which of a positive electrode or a negative electrode.

Two terminals in FIG. 6F are connected to a charger, and a secondarybattery 40 is charged. As the charge of the secondary battery 40proceeds, a potential difference between electrodes increases. In FIG.6F, electrons flow from one terminal outside the secondary battery 40 tothe positive electrode current collector 12; thus, current flows fromthe positive electrode a current collector 12 to the negative electrodecurrent collector 14 in the secondary battery 40. The positive directionin FIG. 6F is the direction of the current that flows from the negativeelectrode to the other terminal outside the secondary battery 40. Inother words, a current flows in the direction of a flow of a chargingcurrent.

In an example in this embodiment, one rectangle film is folded in halfand two end portions are made to overlap with each other for sealing.However, the shape of the film is not limited to a rectangle and can bea polygon such as a triangle, a square, or a pentagon or any symmetricshape other than a rectangle, such as a circle or a star.

Although an example of a small battery used in a portable informationterminal or the like is described in this embodiment, one embodiment ofthe present invention is not particularly limited to this example.Application to a large battery provided in a vehicle or the like is alsopossible.

Although an example of application to a lithium-ion secondary battery isdescribed in this embodiment, one embodiment of the present invention isnot limited to this example. Application to a variety of secondarybatteries such as a lead storage battery, a lithium-ion polymersecondary battery, a nickel-hydrogen storage battery, a nickel-cadmiumstorage battery, a nickel-iron storage battery, a nickel-zinc storagebattery, a silver oxide-zinc storage battery, a solid-state battery, andan air battery is also possible. Application to a variety of powerstorage devices such as a primary battery, a capacitor, and alithium-ion capacitor is also possible. Furthermore, application to asolar cell, an optical sensor, a touch sensor, a display device, aflexible printed circuit (FPC), an optical film (e.g., a polarizingplate, a retardation plate, a prism sheet, a light reflective sheet, anda light diffusion sheet), and the like is also possible.

Embodiment 2

In this embodiment, an example where two films 11 a and 11 b are formedas illustrated in FIG. 3C and attached to each other to fabricate asecondary battery will be described with reference to FIGS. 6A to 6F andFIGS. 8A to 8D. In addition, an example where two films 23 and 24 areformed as illustrated in FIG. 4B and attached to each other to fabricatea secondary battery will be described with reference to FIGS. 7A to 7Cand FIGS. 9A to 9C.

A bendable secondary battery is not used by itself but is mounted on anelectronic device or the like to be used. For this reason, once theshape of a secondary battery to be mounted on an electronic device isfixed, the shape of the secondary battery does not need to be changed inmany cases.

For example, when a secondary battery bent in one direction at a certainangle is mounted on an electronic device, the shape of the secondarybattery is fixed in that state in many cases. In that case, thesecondary battery does not need to be bent back into shape or bentbackward.

Thus, for a bendable secondary battery, the only stress that needs to beconsidered is that applied when the secondary battery is bent in onlyone direction.

FIG. 8B is an example of a cross-sectional view of the secondary batteryin FIG. 6D taken along line C-D, where the secondary battery isfabricated using the film in FIG. 6A and is bent in one direction. Notethat for simplification of the drawings, the electrolyte solution and astack including the positive electrode current collector on the surfaceof which the positive electrode active material layer is formed, theseparator, the negative electrode current collector on the surface ofwhich the negative electrode active material layer is formed, and thelike are collectively illustrated as the components 70 in the battery.

When the secondary battery is bent as illustrated in FIG. 8B,compressive stress is applied to the film 72 that covers the bottomsurface of the secondary battery whereas tensile stress is applied tothe film 71 that covers the top surface of the secondary battery. Byforming a pattern of depressions or projections on surfaces of the film71 as illustrated in FIG. 8B, influence of distortion can be reduced tobe acceptable even when the tensile stress is applied to the film 71.For this reason, the secondary battery can change its form such that theexterior body on the side closer to the center of curvature has acurvature radius greater than or equal to 10 mm, preferably greater thanor equal to 30 mm.

FIG. 9B is an example of a cross-sectional view of the secondary batteryin FIG. 7B taken along line G-H, where the secondary battery isfabricated using the film in FIG. 7A and is bent in one direction. Notethat for simplification of the drawings, the electrolyte solution and astack including the positive electrode current collector on the surfaceof which the positive electrode active material layer is formed, theseparator, the negative electrode current collector on the surface ofwhich the negative electrode active material layer is formed, and thelike are collectively illustrated as the components 70 in the battery.

In FIG. 8B, only the film 71 that covers the top surface of thesecondary battery has a pattern of depressions or projections on itssurfaces; however, one embodiment of the present invention is notlimited thereto, and embossing may be performed not only on the film 71but also on the film 72 that covers the bottom surface of the secondarybattery to which compressive stress is applied, as illustrated in FIG.9B. When both of the films 71 and 72 have a pattern of depressions orprojections, influence of distortion can be further reduced. The film 72that covers the bottom surface of the secondary battery to whichcompressive stress is applied may have a geometric pattern in whichlines slanted in two directions cross each other, a vertical stripepattern 1301 illustrated in FIG. 10A, or a horizontal stripe pattern1302 illustrated in FIG. 10B.

Since tensile stress is applied to the film 71 that covers the topsurface of the secondary battery, it is desirable to perform embossingsuch that the film easily extends. Embossing with a narrow pitchincreases the surface area of a film, so that the film easily extendswhen being bent. Thus, to bend a film at a small radius of curvature, anarrow embossing pitch is preferably employed as illustrated in FIG. 8Cand FIG. 9C rather than that illustrated in FIG. 8A or FIG. 9A.

In FIGS. 9A to 9C, two films with an optimized embossing depth and anoptimized embossing pitch are used for sealing of the secondary battery.However, one film which is subjected to both embossing optimized forrelieving tensile stress and embossing optimized for relievingcompressive stress may be used, in which case embossing optimized forrelieving tensile stress is performed on half of the film and embossingoptimized for relieving compressive stress is performed on the otherhalf. After that, the film is folded along a boundary between the regionsubjected to embossing optimized for relieving tensile stress and theregion subjected to embossing optimized for relieving compressive stresssuch that end portions of the film are aligned, and sealing isperformed.

FIG. 8D is an enlarged view of the film 72 in a region surrounded by adotted line in each of FIG. 8A and FIG. 9A. A stacked-layer film asillustrated in FIG. 8D can be used as the film that covers the bottomsurface of the secondary battery. For example, a film 72 a made ofpolypropylene, a film 72 b made of aluminum, and a film 72 c made ofnylon can be used.

The film 11 a with optimized embossing depth and optimized embossingpitch and the non-embossed film 11 b, or the films 23 and 24 withoptimized embossing depth and optimized embossing pitch are prepared,and then a stack including the positive electrode current collector 12,the separator 13, and the negative electrode current collector 14illustrated in FIG. 6B is provided between the two films as inEmbodiment 1. The positive electrode active material layer 18 is formedon part of a surface of the positive electrode current collector 12, andthe negative electrode active material layer 19 is formed on part of asurface of the negative electrode current collector 14. After that, thelead electrode 16 is electrically connected to each of the protrudingportions of the positive electrode current collector 12 and the negativeelectrode current collector 14 by ultrasonic welding or the like. Then,three sides of the films are sealed by thermocompression bonding to forma bag with one side left open for introduction of an electrolytesolution. In a reduced-pressure atmosphere or an inert atmosphere, adesired amount of electrolyte solution is introduced in the bag. Theside of the film which has not been subjected to thermocompressionbonding is sealed by thermocompression bonding, whereby a secondarybattery is completed.

Although the case where a secondary battery mounted on an electronicdevice or the like is bent in one direction and its shape is fixed isdescribed in this embodiment, the secondary battery bent in onedirection may be almost straightened and bent repeatedly.

Embodiment 3

In this embodiment, examples where a plurality of stacks that are partlydifferent from those in Embodiment 1 are packed in the folded film 11will be described with reference to FIGS. 11A to 11E, FIGS. 12A to 12C,and FIGS. 13A to 13C.

The stacks in this embodiment may be provided between the embossed film11 a and the non-embossed film 11 b or between the embossed films 23 and24 described in Embodiment 2.

FIG. 11A is a top view of the positive electrode current collector 12.FIG. 11B is a top view of the separator 13. FIG. 11C is a top view ofthe negative electrode current collector 14. FIG. 11D is a top view ofthe sealing layer 15 and the lead electrode 16. FIG. 11E is a top viewof the film 11.

The dimensions of the positive electrode current collector, the negativeelectrode current collector, and the separator are substantially thesame in FIGS. 11A to 11E. A region 21 surrounded by a chain line in FIG.11E has substantially the same dimensions as the separator in FIG. 11B.A region between a dotted line in FIG. 11E and an end portion is thethermocompression-bonded region 17.

FIG. 12A illustrates an example where a positive electrode activematerial layer 18 is provided on both surfaces of the positive electrodecurrent collector 12. Specifically, the negative electrode currentcollector 14, the negative electrode active material layer 19, theseparator 13, the positive electrode active material layer 18, thepositive electrode current collector 12, the positive electrode activematerial layer 18, the separator 13, the negative electrode activematerial layer 19, and the negative electrode current collector 14 arestacked in this order. FIG. 12B is a cross-sectional view of thestacked-layer structure taken along a plane 80.

Note that although FIG. 12A illustrates an example where two separatorsare used, the following structure may be employed: one separator isfolded and two edges are sealed to form a bag, and the positiveelectrode current collector 12 is provided in the bag. The positiveelectrode active material layer 18 is formed on both surfaces of thepositive electrode current collector 12 provided in the bag-likeseparator.

The negative electrode active material layer 19 may be provided on bothsurfaces of the negative electrode current collector 14. In thesecondary battery illustrated in FIG. 12C, three negative electrodecurrent collectors 14 whose both surfaces are provided with the negativeelectrode active material layer 19, four positive electrode currentcollectors 12 whose both surfaces are provided with the positiveelectrode active material layer 18, and eight separators 13 aresandwiched between two negative electrode current collectors 14 in eachof which one surface is provided with the negative electrode activematerial layer 19. In this case, four bag-like separators can be usedinstead of eight separators.

The capacity of the secondary battery can be increased by increasing thenumber of the stacks. In addition, when the positive electrode activematerial layer 18 is provided on both surfaces of the positive electrodecurrent collector 12 and the negative electrode active material layer 19is provided on both surfaces of the negative electrode current collector14, the thickness of the secondary battery can be made small.

FIG. 13A illustrates a secondary battery in which the positive electrodeactive material layer 18 is provided on one surface of the positiveelectrode current collector 12 and the negative electrode activematerial layer 19 is provided on one surface of the negative electrodecurrent collector 14. Specifically, the negative electrode activematerial layer 19 is provided on one surface of the negative electrodecurrent collector 14 and the separator 13 is stacked on and in contactwith the negative electrode active material layer 19. On a surface ofthe separator 13 remote from the negative electrode active materiallayer 19, the positive electrode active material layer 18 that isprovided on one surface of the positive electrode current collector 12is provided. On the other surface of the positive electrode currentcollector 12, another positive electrode current collector 12 whose onesurface is provided with the positive electrode active material layer 18is provided. Note that the positive electrode current collectors 12 areprovided such that the surfaces remote from the positive electrodeactive material layers 18 face each other. Another separator 13 isstacked thereon, and the negative electrode active material layer 19provided on one surface of the negative electrode current collector 14is stacked on and in contact with the separator. FIG. 13B is across-sectional view of the stacked-layer structure in FIG. 13A, takenalong a plane 90.

Although two separators are used in FIG. 13A, the following structuremay be employed: one separator is folded and two edges are sealed toform a bag, and two positive electrode current collectors 12 in each ofwhich one surface is provided with the positive electrode activematerial layer 18 are provided in the bag.

In FIG. 13C, a plurality of the stacks illustrated in FIG. 13A arestacked. In FIG. 13C, the negative electrode current collectors 14 areprovided such that the surfaces remote from the negative electrodeactive material layers 19 face each other, and the positive electrodecurrent collectors 12 are provided such that the surfaces remote fromthe positive electrode active material layers 18 face each other. InFIG. 13C, twelve positive electrode current collectors 12, twelvenegative electrode current collectors 14, and twelve separators 13 arestacked.

A secondary battery with a structure in which the positive electrodeactive material layer 18 is provided on one surface of the positiveelectrode current collector 12 and the negative electrode activematerial layer 19 is provided on one surface of the negative electrodecurrent collector 14 is thicker than a secondary battery with astructure in which the positive electrode active material layer 18 isprovided on both surfaces of the positive electrode current collector 12and the negative electrode active material layer 19 is provided on bothsurfaces of the negative electrode current collector 14, because thenumber of the current collectors is larger. However, the surface of thepositive electrode current collector 12 on which the positive electrodeactive material layer 18 is not provided faces the surface of anotherpositive electrode current collector 12 on which the positive electrodeactive material layer 18 is not provided; as a result, metal surfacesare in contact with each other. Similarly, the surface of the negativeelectrode current collector 14 on which the negative electrode activematerial layer 19 is not provided faces the surface of another negativeelectrode current collector 14 on which the negative electrode activematerial layer 19 is not provided; as a result, metal surfaces are incontact with each other. The metal surfaces easily slide on each otherowing to the low friction. Since the metal surfaces in the secondarybattery slide on each other at the time of bending, the secondarybattery is easily bent.

The protruding portions of the positive electrode current collector 12and the negative electrode current collector 14 are also referred to astab portions. The tab portions of the positive electrode currentcollector 12 and the negative electrode current collector 14 are easilycut when the secondary battery is bent. This is because the tab portionsare long and narrow protrusions and the stress is likely to be appliedto the roots of the tab portions.

In the structure in which the positive electrode active material layer18 is provided on one surface of the positive electrode currentcollector 12 and the negative electrode active material layer 19 isprovided on one surface of the negative electrode current collector 14,there are a surface where the positive electrode current collectors 12are in contact with each other and a surface where the negativeelectrode current collectors 14 are in contact with each other. Thesurface where the current collectors are in contact with each other haslow friction resistance and thus easily relieves the stress due to thedifference in radius of curvature that occurs when the battery ischanged in shape. Furthermore, the total thickness of the tab portion islarge in the structure in which the positive electrode active materiallayer 18 is provided on one surface of the positive electrode currentcollector 12 and the negative electrode active material layer 19 isprovided on one surface of the negative electrode current collector 14;thus, the stress is distributed as compared with the case of thestructure in which the positive electrode active material layer 18 isprovided on both surfaces of the positive electrode current collector 12and the negative electrode active material layer 19 is provided on bothsurfaces of the negative electrode current collector 14, and the tabportion is less likely to be cut.

In the case of thus stacking layers, the positive electrode currentcollectors 12 are all fixed and electrically connected at a time byultrasonic welding. Furthermore, when ultrasonic welding is performedwith the positive electrode current collectors 12 overlapping with alead electrode, they can be electrically connected efficiently.

Ultrasonic welding can be performed in such a manner that ultrasonicwaves are applied to the tab portion of the positive electrode currentcollector placed so as to overlap with a tab portion of another positiveelectrode current collector, while pressure is applied thereto.

Embodiment 4

In this embodiment, examples of electronic devices incorporating thelithium-ion secondary battery described in Embodiment 1 or 2 will bedescribed with reference to FIGS. 14A to 14H, FIGS. 15A to 15C, andFIGS. 16A and 16B. In addition, Embodiment 3 can be used in combination.

The secondary battery fabricated according to any of Embodiments 1 to 3includes, as an exterior body, a thin film having flexibility and thuscan be bonded to a support structure body with a curved surface and canchange its form reflecting the curved surface of a region of the supportstructure body that has a large radius of curvature.

Examples of electronic devices each using a flexible power storagedevice are as follows: display devices (also referred to as televisionsor television receivers) such as head-mounted displays and goggle typedisplays, desktop personal computers, notebook personal computers,monitors for computers or the like, cameras such as digital cameras ordigital video cameras, digital photo frames, electronic notebooks,e-book readers, electronic translators, toys, audio input devices suchas microphones, electric shavers, electric toothbrushes, high-frequencyheating appliances such as microwave ovens, electric rice cookers,electric washing machines, electric vacuum cleaners, water heaters,electric fans, hair dryers, air-conditioning systems such ashumidifiers, dehumidifiers, and air conditioners, dishwashers, dishdryers, clothes dryers, futon dryers, electric refrigerators, electricfreezers, electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools, alarm devices such as smokedetectors, gas alarm devices, and security alarm devices, industrialrobots, health equipment and medical equipment such as hearing aids,cardiac pacemakers, X-ray equipment, radiation counters, electricmassagers, and dialyzers, mobile phones (also referred to as mobilephone devices or cell phones), portable game machines, portableinformation terminals, lighting devices, headphone stereos, stereos,remote controls, clocks such as table clocks and wall clocks, cordlessphone handsets, transceivers, pedometers, calculators, portable orstationary music reproduction devices such as digital audio players, andlarge game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 14A illustrates an example of a mobile phone. A mobile phone 7400includes a display portion 7402 incorporated in a housing 7401, anoperation button 7403, an external connection port 7404, a speaker 7405,a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 14B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by external force, the power storage device7407 included in the mobile phone 7400 is also bent. FIG. 14Cillustrates the bent power storage device 7407. The power storage device7407 is a laminated storage battery (also referred to as a layeredbattery or a film-covered battery). The power storage device 7407 isfixed while being bent. Note that the power storage device 7407 includesa lead electrode 7408 electrically connected to a current collector7409. For example, a film serving as an exterior body of the powerstorage device 7407 is embossed, so that the power storage device 7407has high reliability even when bent. The mobile phone 7400 may furtherbe provided with a slot for insertion of a SIM card, a connector portionfor connecting a USB device such as a USB memory.

FIG. 14D illustrates an example of a mobile phone that can be bent. Whenbent to be put around a forearm, the mobile phone can be used as abangle-type mobile phone as in FIG. 14E. A mobile phone 7100 includes ahousing 7101, a display portion 7102, an operation button 7103, and apower storage device 7104. FIG. 14F illustrates the power storage device7104 that can be bent. When the mobile phone is worn on a user's armwhile the power storage device 7104 is bent, the housing changes itsform and the curvature of a part or the whole of the power storagedevice 7104 is changed. Specifically, a part or the whole of the housingor the main surface of the power storage device 7104 is changed in therange of radius of curvature from 10 mm to 150 mm. Note that the powerstorage device 7104 includes a lead electrode 7105 that is electricallyconnected to a current collector 7106. For example, pressing isperformed to form a plurality of projections and depressions on asurface of the film serving as the exterior body of the power storagedevice 7104, and retains high reliability even when the power storagedevice 7104 is bent many times with different curvatures. The mobilephone 7100 may further be provided with a slot for insertion of a SIMcard, a connector portion for connecting a USB device such as a USBmemory. When a center portion of the mobile phone illustrated in FIG.14D is folded, a form illustrated in FIG. 14G can be obtained. When acenter portion of the mobile phone is further folded so that endportions of the mobile phone overlap with each other as illustrated inFIG. 14H, the mobile phone can be reduced in size so as to be put in,for example, a pocket of clothes a user wears. As described above, themobile phone illustrated in FIG. 14D can be changed in form in more thanone way, and it is desirable that at least the housing 7101, the displayportion 7102, and the power storage device 7104 have flexibility inorder to change the form of the mobile phone.

FIG. 15A illustrates an example of a vacuum cleaner. By being providedwith a secondary battery, the vacuum cleaner can be cordless. To securea dust collecting space for storing vacuumed dust inside the vacuumcleaner, a space occupied by a power storage device 7604 is preferablyas small as possible. For this reason, it is useful to provide the thinpower storage device 7604 that can be bent, between the outside surfaceand the dust collecting space.

The vacuum cleaner 7600 is provided with operation buttons 7603 and thepower storage device 7604. FIG. 15B illustrates the power storage device7604 that is capable of being bent. A film serving as an exterior bodyof the power storage device 7604 is embossed, so that the power storagedevice 7604 has high reliability even when bent. The power storagedevice 7604 includes a lead electrode 7601 electrically connected to anegative electrode and a lead electrode 7602 electrically connected to apositive electrode.

As another example of the power storage device 7604 where two leadelectrodes are exposed from one short side of an exterior body, a powerstorage device 7605 that is capable of being bent is illustrated in FIG.15C. The power storage device 7605 has a structure in which a currentcollector or a lead electrode is exposed from each of two short sides ofan exterior body. A film serving as the exterior body of the powerstorage device 7605 may also be embossed, in which case the powerstorage device 7605 can be bent and have high reliability.

The thin power storage device 7604 can be fabricated by the method forfabricating a laminated secondary battery that is described in any ofEmbodiments 1 to 3.

The thin power storage device 7604 has a laminated structure and is bentand fixed. The vacuum cleaner 7600 includes a display portion 7606 thatdisplays, for example, the remaining amount of power in the thin powerstorage device 7604. A display area of the display portion 7606 iscurved to fit the shape of the outer surface of the vacuum cleaner. Thevacuum cleaner includes a connection cord for being connected to areceptacle. When the thin power storage device 7604 is charged to havesufficient power, the connection cord can be removed from the receptacleto use the vacuum cleaner. The thin power storage device 7604 may becharged wirelessly without using the connection cord.

The use of power storage devices that can be bent in vehicles enablesproduction of next-generation clean energy vehicles such as hybridelectric vehicles (HEVs), electric vehicles (EVs), and plug-in hybridelectric vehicles (PHEVs). Moreover, power storage devices that can bebent can also be used in moving objects such as agricultural machines,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, electric carts, boats or ships, submarines,aircrafts such as fixed-wing aircrafts and rotary-wing aircrafts,rockets, artificial satellites, space probes, planetary probes, andspacecrafts.

FIGS. 16A and 16B each illustrate an example of a vehicle fabricatedaccording to one embodiment of the present invention. An automobile 8100illustrated in FIG. 16A is an electric vehicle that runs on the power ofan electric motor. Alternatively, the automobile 8100 is a hybridelectric vehicle capable of driving using either the electric motor orthe engine as appropriate. In the case of providing a laminatedsecondary battery in the vehicle, a battery module including a pluralityof laminated secondary batteries is placed in one place or more than oneplace. According to one embodiment of the present invention, a powerstorage device itself can be made more compact and lightweight, and forexample, when the power storage device having a curved surface isprovided on the inside of a tire of a vehicle, the vehicle can be ahigh-mileage vehicle. Furthermore, a power storage device that can havevarious shapes can be provided in a small space in a vehicle, whichallows a space in a trunk and a space for riders to be secured. Theautomobile 8100 includes the power storage device. The power storagedevice is used not only to drive the electric motor, but also to supplyelectric power to a light-emitting device such as a headlight 8101 or aroom light (not illustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8100. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8100, such as a navigation system.

FIG. 16B illustrates an automobile 8200 including the power storagedevice. The automobile 8200 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.16B, the power storage device included in the automobile 8200 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. For example, with the use of a plug-intechnique, the power storage device included in the automobile 8200 canbe charged by being supplied with electric power from outside. Thecharging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween two vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the degree offlexibility in place where the power storage device can be provided isincreased and thus a vehicle can be designed efficiently. Furthermore,according to one embodiment of the present invention, the power storagedevice itself can be made more compact and lightweight as a result ofimproved characteristics of the power storage device. The compact andlightweight power storage device contributes to a reduction in theweight of a vehicle, and thus increases the driving radius. Furthermore,the power storage device included in the vehicle can be used as a powersource for supplying electric power to products other than the vehicle.In such a case, the use of a commercial power source can be avoided atpeak time of electric power demand.

This embodiment can be freely combined with any one of Embodiments 1 to3.

Note that what is described (or part thereof) in one embodiment can beapplied to, combined with, or replaced with different contents in theembodiment and/or what is described (or part thereof) in anotherembodiment or other embodiments.

Note that in each embodiment, what are described in the embodiment arecontents described with reference to a variety of diagrams or contentsdescribed with text described in this specification.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment with another part of the diagram, adifferent diagram (or may be part of the different diagram) illustratedin the embodiment, and/or a diagram (or may be part of the diagram)illustrated in another embodiment or other embodiments, much morediagrams can be formed.

Note that contents that are not specified in any drawing or text in thespecification can be excluded from one embodiment of the invention.Alternatively, when the range of a value that is defined by the maximumand minimum values is described, part of the range is appropriatelynarrowed or part of the range is removed, whereby one embodiment of theinvention excluding part of the range can be constituted. In thismanner, it is possible to specify the technical scope of one embodimentof the present invention so that a conventional technology is excluded,for example.

As a specific example, a diagram of a circuit including first to fifthtransistors is illustrated. In that case, it can be specified that thecircuit does not include a sixth transistor in the invention. It can bespecified that the circuit does not include a capacitor in theinvention. It can be specified that the circuit does not include a sixthtransistor with a particular connection structure in the invention. Itcan be specified that the circuit does not include a capacitor with aparticular connection structure in the invention. For example, it can bespecified that a sixth transistor whose gate is connected to a gate ofthe third transistor is not included in the invention. For example, itcan be specified that a capacitor whose first electrode is connected tothe gate of the third transistor is not included in the invention.

As another specific example, the description of a value, “a voltage ispreferably higher than or equal to 3 V and lower than or equal to 10 V”is given. In that case, for example, it can be specified that the casewhere the voltage is higher than or equal to −2 V and lower than orequal to 1 V is excluded from one embodiment of the invention. Forexample, it can be specified that the case where the voltage is higherthan or equal to 13 V is excluded from one embodiment of the invention.Note that, for example, it can be specified that the voltage is higherthan or equal to 5 V and lower than or equal to 8 V in the invention.For example, it can be specified that the voltage is approximately 9 Vin the invention. For example, it can be specified that the voltage ishigher than or equal to 3 V and lower than or equal to 10 V but is not 9V in the invention. Note that even when the description “a value ispreferably in a certain range” or “a value preferably satisfies acertain condition” is given, the value is not limited to thedescription. In other words, a description of a value that includes aterm “preferable”, “preferably”, or the like does not necessarily limitthe value.

As another specific example, the description “a voltage is preferred tobe 10 V” is given. In that case, for example, it can be specified thatthe case where the voltage is higher than or equal to −2 V and lowerthan or equal to 1 V is excluded from one embodiment of the invention.For example, it can be specified that the case where the voltage ishigher than or equal to 13 V is excluded from one embodiment of theinvention.

As another specific example, the description “a film is an insulatingfilm” is given to describe a property of a material. In that case, forexample, it can be specified that the case where the insulating film isan organic insulating film is excluded from one embodiment of theinvention. For example, it can be specified that the case where theinsulating film is an inorganic insulating film is excluded from oneembodiment of the invention. For example, it can be specified that thecase where the insulating film is a conductive film is excluded from oneembodiment of the invention. For example, it can be specified that thecase where the insulating film is a semiconductor film is excluded fromone embodiment of the invention.

As another specific example, the description of a stacked structure, “afilm is provided between an A film and a B film” is given. In that case,for example, it can be specified that the case where the film is astacked film of four or more layers is excluded from the invention. Forexample, it can be specified that the case where a conductive film isprovided between the A film and the film is excluded from the invention.

Note that in this specification and the like, it may be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), and the like are connected are not specified. In otherwords, one embodiment of the invention is clear even when connectionportions are not specified. Furthermore, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where the number ofportions to which the terminal is connected may be more than one, it isnot necessary to specify the portions to which the terminal isconnected. Therefore, it may be possible to constitute one embodiment ofthe invention by specifying only portions to which some of terminals ofan active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), and the like are connected.

Note that in this specification and the like, it may be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it may bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention is clear. Moreover, it can be determined that one embodimentof the present invention whose function is specified is disclosed inthis specification and the like. Therefore, when a connection portion ofa circuit is specified, the circuit is disclosed as one embodiment ofthe invention even when a function is not specified, and one embodimentof the invention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Note that in this specification and the like, part of a diagram or textdescribed in one embodiment can be taken out to constitute oneembodiment of the invention. Thus, in the case where a diagram or textrelated to a certain portion is described, the contents taken out frompart of the diagram or the text are also disclosed as one embodiment ofthe invention, and one embodiment of the invention can be constituted.The embodiment of the present invention is clear. Therefore, forexample, in a diagram or text in which one or more active elements(e.g., transistors or diodes), wirings, passive elements (e.g.,capacitors or resistors), conductive layers, insulating layers,semiconductor layers, organic materials, inorganic materials,components, devices, operating methods, manufacturing methods, or thelike are described, part of the diagram or the text is taken out, andone embodiment of the invention can be constituted. For example, from acircuit diagram in which N circuit elements (e.g., transistors orcapacitors; N is an integer) are provided, it is possible to take out Mcircuit elements (e.g., transistors or capacitors; M is an integer,where M<N) and constitute one embodiment of the invention. For anotherexample, it is possible to take out M layers (M is an integer, whereM<N) from a cross-sectional view in which N layers (N is an integer) areprovided and constitute one embodiment of the invention. For anotherexample, it is possible to take out M elements (M is an integer, whereM<N) from a flow chart in which N elements (N is an integer) areprovided and constitute one embodiment of the invention. For anotherexample, it is possible to take out some given elements from a sentence“A includes B, C, D, E, or F” and constitute one embodiment of theinvention, for example, “A includes B and E”, “A includes E and F”, “Aincludes C, E, and F”, or “A includes B, C, D, and E”.

Note that in the case where at least one specific example is describedin a diagram or text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the present invention is clear.

Note that in this specification and the like, what is illustrated in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when certain contents are described in adiagram, the contents are disclosed as one embodiment of the inventioneven when the contents are not described with text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the present invention is clear.

Example 1

In this example, comparison results between friction of an electrode inwhich an active material layer is provided on both surfaces of a currentcollector and that of an electrode in which an active material layer isprovided on one surface of a current collector will be described withreference to FIGS. 17A to 17C and FIG. 18.

For the comparison of friction, first, a plurality of electrodes werestacked over a glass plate and an electrode in the middle was pulledout.

An actual secondary battery includes, in a region surrounded by anexterior body, a first current collector a surface of which is providedwith a negative electrode active material layer, a second currentcollector a surface of which is provided with a positive electrodeactive material layer, and a third current collector a surface of whichis provided with a negative electrode active material layer. The firstto current collectors partly overlap with each other. A separator isprovided between the first current collector and the second currentcollector and between the second current collector and the third currentcollector.

In this example, a stack having the same structure as the abovestructure in a secondary battery was used for the experiment.

The maximum value of friction was measured in the following manner: thefirst to third current collectors were stacked over the glass plate anda load of 42 g/cm² was applied thereto, and then the current collectorthe surface of which is provided with the positive electrode activematerial layer was pulled out in the horizontal direction by a loadmeasuring instrument (RX-2 manufactured by AIKOH ENGINEERING CO., LTD.).

FIG. 17A is a schematic cross-sectional view of a sample including anegative electrode in which an active material layer is provided on onesurface of a current collector. As illustrated in FIG. 17A, a negativeelectrode active material layer 102 a, a negative electrode currentcollector 103 a, a negative electrode current collector 103 b, anegative electrode active material layer 102 b, a separator 104, apositive electrode active material layer 105 a, a positive electrodecurrent collector 106, a positive electrode active material layer 105 b,the separator 104, a negative electrode active material layer 107 a, anegative electrode current collector 108 a, a negative electrode currentcollector 108 b, and a negative electrode active material layer 107 bwere stacked over a glass plate 101 in this order. Then, a positiveelectrode (the positive electrode active material layer 105 a, thepositive electrode current collector 106, and the positive electrodeactive material layer 105 b) was pulled out in a horizontal direction110 by a clip 120 while a load 109 was applied, so that friction wasmeasured.

FIG. 17B is a schematic cross-sectional view of a sample including anegative electrode in which an active material layer is provided on bothsurfaces of a current collector. As illustrated in FIG. 17B, thenegative electrode active material layer 102 a, a negative electrodecurrent collector 103, the negative electrode active material layer 102b, the separator 104, the positive electrode active material layer 105a, the positive electrode current collector 106, the positive electrodeactive material layer 105 b, the separator 104, the negative electrodeactive material layer 107 a, a negative electrode current collector 108,and the negative electrode active material layer 107 b were stacked overthe glass plate 101 in this order. Then, a positive electrode (thepositive electrode active material layer 105 a, the positive electrodecurrent collector 106, and the positive electrode active material layer105 b) was pulled out in a horizontal direction 110 by a clip 120 whilea load 109 was applied, so that friction was measured.

Note that the samples in FIGS. 17A and 17B each include a positiveelectrode in which an active material layer is provided on both surfacesof a current collector. Thus, the positive electrode includes thepositive electrode active material layer 105 a, the positive electrodecurrent collector 106, and the positive electrode active material layer105 b.

FIG. 17C is a photograph of the comparative experiment of friction.

The samples in FIGS. 17A and 17B were also compared under the conditionswhere the stack was dried and where the stack was wet with anelectrolyte solution. The samples were measured three times for eachcondition. FIG. 18 shows the measurement results.

The results in FIG. 18 show that the secondary battery having theinternal structure illustrated in FIG. 17A was easily bent under boththe dry and wet conditions. This is because the metal surfaces incontact with each other easily slide on each other owing to the lowfriction. The results show that the secondary battery including thenegative electrode in which the active material layer is provided on onesurface of the current collector is easily bent.

Example 2

In this example, experimental results will be described with referenceto FIGS. 19A to 19D, FIG. 20, and FIGS. 21A to 21C. In the experiment, awater barrier property examination test was conducted after a thinsecondary battery was repeatedly bent with the use of an apparatus shownin FIGS. 21A to 21C to determine whether an exterior body of thesecondary battery was damaged by the bending.

Whether the exterior body was damaged or not was determined not only byvisual inspection but also by measurement of the amount of moisture inthe secondary battery with a Karl Fischer moisture meter. In the casewhere the amount of moisture exceeded 100 ppm, the sample was determinedto have insufficient water barrier property. When the exterior body isheavily damaged by bending and cracked, for example, moisture entersthrough the damaged portion; thus, the amount of moisture in thesecondary battery increases.

As the Karl Fischer moisture meter, an apparatus using a coulometrictitration method (MKC-610-DT manufactured by KYOTO ELECTRONICSMANUFACTURING CO., LTD.) was used.

First, twelve samples were fabricated. The twelve samples had differentlengths and widths as illustrated in FIGS. 19A to 19D and FIG. 20. Inaddition, the number of stacked layers, that is, the thickness of thesample, and the radius of curvature of bending were made different.

The samples were fabricated according to Embodiments 1 to 3 in the samemanner as the secondary battery whose exterior body is an embossed film.Note that propylene carbonate was provided in a space surrounded by theexterior body of the secondary battery instead of an electrolytesolution, and the amount of moisture in propylene carbonate wasmeasured. For this reason, the samples in this example did not functionas secondary batteries and were not able to be charged and discharged;however, charging and discharging are possible if an electrolytesolution is used instead of propylene carbonate. The design capacitiesin such a case were calculated and shown in Table 1.

TABLE 1 Number of Cell Cell Cell Design Radius of Water stackedthickness length width capacity curvature of barrier layers [mm] [mm][mm] [mAh] bending [mm] property 2 1.5 75 60 110 40 OK 6 2.5 75 60 33040 OK 10 3.4 75 60 550 40 OK 16 4.7 75 60 880 40 NG 6 2.5 75 60 330 30OK 6 2.5 75 60 330 60 OK 6 2.5 75 60 330 100 OK 6 2.5 38 60 86 40 OK 62.5 89 60 422 40 NG 4 2.0 103 60 343 40 NG 6 2.5 103 60 515 40 NG 6 2.575 42 185 40 OK

Table 2 shows the total thickness of the sample for the correspondingnumber of stacked current collectors provided in the region surroundedby the exterior body.

TABLE 2 2 layers 1.5 mm 4 layers 2.0 mm 6 layers 2.5 mm 10 layers  3.4mm 16 layers  4.7 mm

Three samples were fabricated for each of the twelve kinds of samplesand 10000-time repetitive bending was performed with a bending tester.After that, the samples were held together with water in a container(constant temperature bath) having a pressure adjusting mechanism at120° C. for 24 hours, and then, the amount of moisture in each samplewas measured with the Karl Fischer moisture meter. Table 1 shows themeasurement results. In Table 1, “NG” means that at least one out ofthree samples had an amount of moisture exceeding 100 ppm.

Note that FIGS. 21A and 21B are photographs of a sample bent with thebending tester. The bending tester can set the radius of curvature. Inthis example, the minimum radius of curvature of bending was set to 30mm, 40 mm, 60 mm, or 100 mm as shown in Table 1.

The results in Table 1 show that a cell with a length of 75 mm orshorter and a thickness of 3.4 mm or smaller had slight damage of theexterior body and thus retained the water barrier property even after10000-time repetitive bending.

FIG. 21C is a photograph of the appearance of a bending tester 1100. Afabricated lithium-ion secondary battery 1200 was placed on the bendingtester 1100. Note that the lithium-ion secondary battery 1200 wassandwiched between two holding plates 1101, and the lithium-ionsecondary battery 1200 is shown by a broken line in FIG. 21C. Thebending tester 1100 included a cylindrical supporting body (notillustrated) with a radius of curvature of 40 mm extending in thedirection perpendicular to the arm 1102 under the lithium-ion secondarybattery 1200 in a center portion. The bending tester 1100 also includesarms 1102 extending in the right and left directions. End portions ofthe arms 1102 are mechanically connected to holding plates 1101. Bymoving the end portions of the arms 1102 up or down, the holding plates1101 can be bent along the supporting body. The bending test of thelithium-ion secondary battery 1200 was performed with the lithium-ionsecondary battery 1200 sandwiched between the two holding plates 1101.Thus, moving the end portions of the arms 1102 up or down allows thelithium-ion secondary battery 1200 to be bent repeatedly along thecylindrical supporting body. Specifically, lowering the end portions ofthe arms 1102 permits the lithium-ion secondary battery 1200 to be bentwith a radius of curvature of 40 mm. Since the lithium-ion secondarybattery 1200 was bent repeatedly while being sandwiched between the twoholding plates 1101, unnecessary force except repetitive bending forcewas able to be prevented from being applied to the lithium-ion secondarybattery 1200. Furthermore, repetitive bending force was able to beuniformly applied to the whole lithium-ion secondary battery 1200.

Example 3

In this example, measurement results of a load necessary for bendingeach of a secondary battery including an electrode in which an activematerial layer is provided on one surface of a current collector and asecondary battery including an electrode in which an active materiallayer is provided on both surfaces of a current collector will bedescribed with reference to FIGS. 22A and 22B and FIGS. 23A and 23B.

First, a sample of the secondary battery including the electrode inwhich the active material layer was provided on one surface of thecurrent collector and a sample of the secondary battery in which theactive material layer was provided on both surfaces of the currentcollector were prepared.

The specific structures of the secondary batteries were as follows. Inthe sample in which an active material layer was provided on one surfaceof a current collector, six positive electrode current collectors 1904in each of which one surface was provided with a positive electrodeactive material layer 1905, four negative electrode current collectors1902 in each of which one surface was provided with a negative electrodeactive material layer 1903, and six separators 1906 were sandwichedbetween two negative electrode current collectors 1902 in each of whichone surface was provided with the negative electrode active materiallayer 1903.

In the sample in which an active material layer was provided on bothsurfaces of a current collector, three positive electrode currentcollectors 1904 whose both surfaces were provided with the positiveelectrode active material layer 1905, two negative electrode currentcollectors 1902 whose both surfaces were provided with the negativeelectrode active material layer 1903, and six separators 1906 weresandwiched between two negative electrode current collectors 1902 ineach of which one surface was provided with the negative electrodeactive material layer 1903.

FIG. 22A is a schematic cross-sectional view of the sample in which anactive material layer was provided on one surface of a currentcollector. As illustrated in FIG. 22A, the negative electrode currentcollector 1902, the negative electrode active material layer 1903, theseparator 1906, the positive electrode active material layer 1905, thepositive electrode current collector 1904, the positive electrodecurrent collector 1904, the positive electrode active material layer1905, the separator 1906, the negative electrode active material layer1903, the negative electrode current collector 1902, the negativeelectrode current collector 1902, the negative electrode active materiallayer 1903, the separator 1906, the positive electrode active materiallayer 1905, the positive electrode current collector 1904, the positiveelectrode current collector 1904, the positive electrode active materiallayer 1905, the separator 1906, the negative electrode active materiallayer 1903, the negative electrode current collector 1902, the negativeelectrode current collector 1902, the negative electrode active materiallayer 1903, the separator 1906, the positive electrode active materiallayer 1905, the positive electrode current collector 1904, the positiveelectrode current collector 1904, the positive electrode active materiallayer 1905, the separator 1906, the negative electrode active materiallayer 1903, and the negative electrode current collector 1902 werestacked in this order to fabricate a secondary battery with a thicknessof approximately 2 mm.

FIG. 22B is a schematic cross-sectional view of the sample in which anactive material layer was provided on both surfaces of a currentcollector. As illustrated in FIG. 22B, the negative electrode currentcollector 1902, the negative electrode active material layer 1903, theseparator 1906, the positive electrode active material layer 1905, thepositive electrode current collector 1904, the positive electrode activematerial layer 1905, the separator 1906, the negative electrode activematerial layer 1903, the negative electrode current collector 1902, thenegative electrode active material layer 1903, the separator 1906, thepositive electrode active material layer 1905, the positive electrodecurrent collector 1904, the positive electrode active material layer1905, the separator 1906, the negative electrode active material layer1903, the negative electrode current collector 1902, the negativeelectrode active material layer 1903, the separator 1906, the positiveelectrode active material layer 1905, the positive electrode currentcollector 1904, the positive electrode active material layer 1905, theseparator 1906, the negative electrode active material layer 1903, andthe negative electrode current collector 1902 were stacked in this orderto fabricate a secondary battery with a thickness of approximately 2 mm.

Next, each sample was sandwiched between jigs 1400 having curvedsurfaces, and a load was applied with a tester 1410 (EZ Graphmanufactured by Shimadzu Corporation) in the following manner: a jig1401 was moved toward a jig 1402 such that the flat sample had adisplacement of 6 mm/min. until the radius of curvature becameapproximately 40 mm. Note that the sample was curved in a directionparallel to a direction in which a lead electrode was extracted.

FIGS. 23A and 23B are schematic views of the jigs 1400 having curvedsurfaces and the tester 1410. In the jigs 1400 having curved surfaces,the jig 1401 in contact with an inner surface of a curved sample 1420has a projection with a radius of curvature of 40 mm, and the jig 1402in contact with an outer surface of the curved sample 1420 has adepression with a radius of curvature of 42 mm.

In FIGS. 23A and 23B, X₁ is the maximum distance between upper surfacesof the jigs 1401 and 1402. In FIG. 23B, X₂ is the minimum distancebetween the upper surfaces of the jigs 1401 and 1402, and ΔX is thedifference between X₁ and X₂, which is the distance through which thejig 1401 actually moves.

The cell was curved in the following manner: the jig 1401 which was tobe in contact with the inner surface of the curved sample and the jig1402 which was to be in contact with the outer surface of the curvedsample were connected to the tester 1410; the sample 1420 was positionedon the jig 1402 which was to be in contact with the outer surface; and aload was applied to the sample 1420 sandwiched between the jigs 1400 ina direction of an arrow (compression direction) in FIG. 23A with thetester 1410. In this manner, a load necessary for bending the sampleuntil the radius of curvature became 40 mm as illustrated in FIG. 23Bwas measured.

FIG. 24 shows the measurement results of loads necessary for curvingsample in which an active material layer was provided on one surface ofa current collector and the sample in which an active material layer wasprovided on both surfaces of a current collector until the radius ofcurvature became 40 mm. The solid line indicates the result for thesample in which an active material layer was provided on one surface ofa current collector, and a dashed-dotted line indicates the result forthe sample in which an active material layer was provided on bothsurfaces of a current collector. Note that “displacement” shown by thelateral axis is the distance through which the jig 1401 in the jigs 1400in contact with the inner surface of the sample moves, which is ΔX inFIG. 23B. The displacement is 0 mm when the distance between the jigs1400 is 40 mm and the sample is not curved, and the displacement 40 mmwhen the contact area of the sample and the jigs 1400 becomes themaximum and the measurement terminates. The displacement can also bereferred to as the displacement amount, the moved distance, and thelike.

As shown in the graph in FIG. 24, a load necessary for curving eachsample starts to rapidly increase at a certain displacement. This isbecause thin portions such as tab portions of a positive electrode and anegative electrode were curved first and then a thick portion includingthe active material layer was curved.

The results shown in FIG. 24 show that the secondary battery includingthe electrode in which the active material layer was provided on onesurface of the current collector was curved with a small load ascompared with the secondary battery including the electrode in which theactive material layer was provided on both surfaces of the currentcollector. This is because the metal surfaces in contact with each othereasily slide on each other owing to the low friction. The results showthat the secondary battery including the negative electrode in which theactive material layer is provided on one surface of the currentcollector is easily curved.

Example 4

In this example, measurement results of loads for curving thin secondarybatteries having different thicknesses will be described.

First, two samples A, two samples B, and two samples C were fabricated.Two layers were stacked in the sample A (thickness: 1.5 mm), six layerswere stacked in the sample B (thickness: 2.5 mm), and ten layers werestacked in the sample C (thickness: 3.4 mm). Note that “two layers werestacked” means two positive electrodes and two negative electrodes werestacked.

The fabrication conditions for the sample in Example 2 with a length of75 mm and a width of 60 mm were employed for the samples A, B, and C,except for the number of stacked layers. Table 3 shows the conditions ofthe samples A, B, and C.

TABLE 3 Radius of curvature Number of Design of stacked Thickness LengthWidth capacity bending Sample layers [mm] [mm] [mm] [mAh] [mm] A 2 1.575 60 110 40 B 6 2.5 75 60 330 40 C 10 3.4 75 60 550 40

Next, each of the samples A, B, and C was sandwiched between jigs 1400having curved surfaces, and a load was applied with the tester 1410 (EZGraph manufactured by Shimadzu Corporation) in the following manner: thejig 1401 was moved toward the jig 1402 such that the flat sample had thedisplacement of 6 mm/min. until the radius of curvature becameapproximately 40 mm. Note that the cell was curved in a directionparallel to a direction in which a lead electrode was extracted.

The load was measured with a tester and jigs similar to those used inExample 3.

FIG. 25A shows the measurement results of loads necessary for curvingthe samples A, B, and C until the radius of curvature became 40 mm. Asolid line indicates the result of the sample A (the number of stackedlayers: 2, thickness: 1.5 mm), a dashed line indicates the result of thesample B (the number of stacked layers: 6, thickness: 2.5 mm), and adotted line indicates the result of the sample C (the number of stackedlayers: 10, thickness: 3.4 mm). For simplification of the graph, FIG.25A shows the results of one sample A, one sample B, and one sample C.

As shown in the graph in FIG. 25A, a load necessary for curving eachsample starts to rapidly increase at a certain displacement. This isbecause thin portions such as tab portions of a positive electrode and anegative electrode were curved first and then a thick portion includingthe active material layer was curved.

Points where the slopes of the curves of the samples A, B, and C in FIG.25A starts to rapidly increase (points where the slopes of the curvesexceed 6 N/mm) are regarded as load values necessary for curving thethick portion including the active material layer. In FIG. 25B, a pointwhere the slope exceeds 6 N/mm is plotted for each of the samples A, B,and C, where a lateral axis represents the thickness of the sample.

As is obvious from FIG. 25B, the thicker the sample is, the more theload is necessary for curving the sample, specifically, the thickportion including the active material layer.

EXPLANATION OF REFERENCE

10: film, 10 a: film, 10 b: film, 11: film, 11 a: film, 11 b: film, 12:positive electrode current collector, 13: separator, 14: negativeelectrode current collector, 15: sealing layer, 16: lead electrode, 17:thermocompression-bonded region, 18: positive electrode active materiallayer, 19: negative electrode active material layer, 20: electrolytesolution, 21: region, 22: film, 23: film, 24: film, 30: adhesive layer,40: secondary battery, 50: film, 51: film, 52: film, 53: embossing roll,54: roll, 55: embossing roll, 56: embossing roll, 57: embossing roll,58: embossing roll, 60: direction, 70: components, 71: film, 72: film,72 a: film, 72 b: film, 72 c: film, 80: plane, 90: plane, 101: glassplate, 102 a: negative electrode active material layer, 102 b: negativeelectrode active material layer, 103: negative electrode currentcollector, 103 a: negative electrode current collector, 103 b: negativeelectrode current collector, 104: separator, 105 a: positive electrodeactive material layer, 105 b: positive electrode active material layer,106: positive electrode current collector, 107 a: negative electrodeactive material layer, 107 b: negative electrode active material layer,108: negative electrode current collector, 108 a: negative electrodecurrent collector, 108 b: negative electrode current collector, 109:load, 110: horizontal direction, 120: clip, 1100: tester, 1101: holdingplate, 1102: arm, 1200: lithium-ion secondary battery, 1301: verticalstripe pattern, 1302: horizontal stripe pattern, 1400: jig, 1401: jig,1402: jig, 1410: tester, 1420: sample, 1700: curved surface, 1701:plane, 1702: curve, 1703: radius of curvature, 1704: center ofcurvature, 1800: center of curvature, 1801: film, 1802: radius ofcurvature, 1803: film, 1804: radius of curvature, 1805: electrodes andelectrolyte solution, 1901: exterior body, 1902: negative electrodecurrent collector, 1903: negative electrode active material layer, 1904:positive electrode current collector, 1905: positive electrode activematerial layer, 1906: separator, 7100: mobile phone, 7101: housing,7102: display portion, 7103: operation button, 7104: power storagedevice, 7105: lead electrode, 7106: current collector, 7400: mobilephone, 7401: housing, 7402: display portion, 7403: operation button,7404: external connection port, 7405: speaker, 7406: microphone, 7407:power storage device, 7408: lead electrode, 7409: current collector,7600: vacuum cleaner, 7601: lead electrode, 7602: lead electrode, 7603:operation button, 7604: power storage device, 7605: power storagedevice, 7606: display portion, 8021: charging device, 8022: cable, 8100:automobile, 8101: headlight, and 8200: automobile.

This application is based on Japanese Patent Application serial no.2014-111114 filed with Japan Patent Office on May 29, 2014, JapanesePatent Application serial no. 2014-133121 filed with Japan Patent Officeon Jun. 27, 2014, Japanese Patent Application serial no. 2014-264017filed with Japan Patent Office on Dec. 26, 2014, and Japanese PatentApplication serial no. 2015-005404 filed with Japan Patent Office onJan. 14, 2015, the entire contents of which are hereby incorporated byreference.

The invention claimed is:
 1. A secondary battery comprising a film,wherein the film has a region with a first pattern of depressions orprojections and a region without the first pattern of the depressions orthe projections, wherein the region with the first pattern comprises aportion having a first thickness and a portion having a secondthickness, wherein sealing is performed using the film, wherein thesecondary battery has flexibility, wherein the secondary battery is amulti-stable flexible secondary battery having at least a first stablecondition and a second stable condition, wherein the first stablecondition is flat, wherein the second stable condition is fixed whilebeing bent, and wherein the secondary battery deforms repeatedly betweenthe first stable condition and the second stable condition.
 2. Thesecondary battery according to claim 1, wherein the depressions or theprojections have a first pitch in the portion having the firstthickness, and wherein the depressions or the projections have a secondpitch in the portion having the second thickness.
 3. The secondarybattery according to claim 2, wherein the first pitch is different fromthe second pitch.
 4. The secondary battery according to claim 2, whereinthe depths of the depressions with the first pitch or the heights of theprojections with the first pitch and the depths of the depressions withthe second pitch or the heights of the projections with the second pitchare smaller than half the thickness of the secondary battery.
 5. Thesecondary battery according to claim 1, comprising a boundary betweenthe region with the first pattern and the region without the firstpattern.
 6. The secondary battery according to claim 1, wherein the filmhas a region with a second pattern of depressions or projectionsdifferent from the first pattern, and wherein the region with the secondpattern comprises a portion having a third thickness and a portionhaving a fourth thickness.
 7. The secondary battery according to claim6, comprising a boundary between the first pattern and the secondpattern.
 8. The secondary battery according to claim 1, comprising atleast a positive electrode active material layer, a negative electrodeactive material layer, and an electrolyte solution in a space inside thefilm which is folded.
 9. The secondary battery according to claim 1,wherein a surface of the film far from a center of curvature is theregion with the first pattern in the second stable condition.
 10. Anelectronic device comprising: a housing; the secondary battery accordingto claim 1, the secondary battery being in the housing; and a displaydevice in the housing, the display device being electrically connectedto the secondary battery.
 11. A secondary battery comprising a film,wherein the film has a region with a first pattern of depressions orprojections and a region without the first pattern of the depressions orthe projections, wherein the region with the first pattern comprises aportion having a first thickness and a portion having a secondthickness, wherein sealing is performed using the film, wherein thedepressions or the projections have a first pitch in the portion havingthe first thickness, and wherein the depressions or the projections havea second pitch in the portion having the second thickness.
 12. Thesecondary battery according to claim 11, comprising a boundary betweenthe region with the first pattern and the region without the firstpattern.
 13. The secondary battery according to claim 11, wherein thefilm has a region with a second pattern of depressions or projectionsdifferent from the first pattern, and wherein the region with the secondpattern comprises a portion having a third thickness and a portionhaving a fourth thickness.
 14. The secondary battery according to claim13, comprising a boundary between the first pattern and the secondpattern.
 15. The secondary battery according to claim 11, comprising atleast a positive electrode active material layer, a negative electrodeactive material layer, and an electrolyte solution in a space inside thefilm which is folded.
 16. The secondary battery according to claim 11,wherein the first pitch is different from the second pitch.
 17. Thesecondary battery according to claim 11, wherein the depths of thedepressions with the first pitch or the heights of the projections withthe first pitch and the depths of the depressions with the second pitchor the heights of the projections with the second pitch are smaller thanhalf the thickness of the secondary battery.
 18. The secondary batteryaccording to claim 11, wherein the secondary battery has flexibility,wherein the secondary battery is a multi-stable flexible secondarybattery having at least a first stable condition and a second stablecondition, wherein the first stable condition is flat, wherein thesecond stable condition is fixed while being bent, wherein the secondarybattery deforms repeatedly between the first stable condition and thesecond stable condition, and wherein a surface of the film far from acenter of curvature is the region with the first pattern in the secondstable condition.
 19. An electronic device comprising: a housing; thesecondary battery according to claim 11, the secondary battery being inthe housing; and a display device in the housing, the display devicebeing electrically connected to the secondary battery.